This invention relates to the field of radio frequency (R.F.) circuits and more particularly to circuits that employ directional couplers and PIN diode switches, and that find application in radio transceivers.
The radiation of undesired electromagnetic signals from a radio transceiver is generally referred to as spurious radiation. Spurious radiation is undesirable because of its potential to interfere with other radio receivers being operated nearby. Since radio receiver circuits usually employ at least one oscillator circuit, commonly known as the local oscillator, a primary source of spurious radiation is the "injection" signal generated by the local oscillator. In addition, circuit non-linearities produce harmonics, or integer multiples of the fundamental local oscillator frequency, which can also be spuriously radiated. Other osciallators, for example a second local osciallator or a clock osciallator for a microprocessor, are also potential sources of spurious radiation, as well as the mixing products of two or more of these oscillators.
Generally, there are two ways in which spurious signals can be radiated. First, the various conductors and components in the transceiver can operate as antennas by directly radiating the spurious signal. Direct radiation can usually be reduced to an acceptable level by electromagnetically shielding the offending circuit, by reducing the length of conductors that carry these signals, and by terminating transmission lines in their characteristic impedance to prevent large standing waves.
The second way in which spurious signals are radiated is by conduction, i.e., by unintentionally creating a conducting path between the source of the spurious signal and the antenna. For example, an amplifier which is connected in a path between the antenna and the source of a spurious signal can conduct in the reverse direction, from output to input; or in the forward direction, from input to output, even though the power to the amplifier is turned off. Conduction through an amplifier in such a manner is not without attenuation; however, if the attenuation is insufficient, a significant amount of spurious energy can be conducted to the antenna.
In FIG. 1, a prior art radio transceiver is illustrated that significantly reduces the amount of local osciallator energy that is conducted to the antenna. A receiver front end 102 includes cascaded band pass filter 104, R.F. amplifier 106, and band pass filter 108. A received signal is picked up by an antenna 110 and coupled to the input of band pass filter 104 by antenna switch 112. The mainline 116 of a directional coupler 118 is connected between the output of band pass filter 108 and the input of a mixer 120. An intermediate frequency (I.F.) stage 122, which includes a crystal filter and an amplifier, is connected to the output of mixer 120. The receiver back end 124, which includes a demodulator and an audio amplifier, is connected to the output of I.F. stage 122. The output of a local oscillator 126 is connected to the input of the coupled arm 127 of directional coupler 118 and the output of the coupled arm is terminated in its characteristic impedance by resistor 128.
Those skilled in the art will understand that directional coupler 118 couples the majority of the local oscillator energy to the input of mixer 120, but very little energy is coupled to the output of band pass filter 108. This significantly reduces the amount of local oscillator energy that is conducted to antenna 110 by flowing backwards through receiver front end 102 and antenna switch 112.
An injection buffer amplifier 130 couples the output of coupled arm 127 to one input of a mixer 132. A transmitter offset oscillator 134, which is modulated by modulator 136, is connected to the second input of mixer 132. The signal at the output of mixer 132 is the transmit signal which is directed to the input of an exciter 138. Those skilled in the art will recognize that the output of mixer 132 includes not only the desired transmit frequency, but also other frequency components that must be removed by exciter stage 138, such as by the use of a band pass filter or a phase lock loop. An R.F. power amplifier 140 is connected between the output of exciter 138 and the transmit input of antenna switch 112.
The particular transceiver architecture illustrated in FIG. 1 is commonly used when the difference or "split" between the transmit and receive frequencies is a fixed frequency. This is typical in transceiver designs that are intended to be operated in repeater systems that, by their nature, must receive and transmit on different frequencies. Thus, if the receiver frequency is changed by changing the frequency of receiver local oscillator 126, no change in the transmitter offset oscillator 134 is required because the transmit frequency is always the receive frequency plus or minus the split frequency. Those skilled in the art will recognize that where the local oscillator frequency is below the receive frequency ("low side injection"), and the transmit frequency is below the receive frequency, the selected output frequency from mixer 132 is the difference of the two input frequencies.
In the receive mode, the power to injection buffer 130, mixer 132, exciter 138 and R.F. power amplifier 140 is turned off. As previously stated, however, conduction of the local oscillator signal can occur through these stages, even though no power is applied. In addition, antenna switch 112 will couple a small amount of spurious energy present at its transmitter input (output of R.F. power amplifier 140) to antenna 110, even though the switch is presently in the receive position, i.e. antenna 110 is connected to receiver front end 102. To reduce this conducted spurious radiation, injection buffer 130 is typically designed to have a large insertion loss in the power off mode, thereby significantly reducing the strength of the local oscillator signal before it reaches the input of mixer 132.
It will be apparent that if the I.F. frequency equals the split frequency (F.sub.I.F. =F.sub.split) then the offset oscillator frequency goes to zero (F.sub.offset =0). Therefore, offset oscillator 134 and mixer 132 could be eliminated and the output of injection buffer 130 would be connected to the input of exciter 138. It would be desirable, however, if injection buffer 130 could also be eliminated without increasing the conduction of spurious local oscillator energy to antenna 110 through exciter 138, R.F. power amplifier 140 and antenna switch 112.
In FIG. 2, a prior art PIN diode antenna or "T/R" switch is illustrated. A transmitter 202 is coupled to an antenna 204 by a coupling capacitor 206 and a PIN diode 208, wherein the cathode of the PIN diode is connected to the antenna. A receiver 210 is also connected to antenna 204 through a coupling capacitor 212 and a one-quarter wavelength transmission line 214. The anode of a PIN diode 216 is connected to the junction of transmission line 214 and coupling capacitor 212, and its cathode is connected to ground. An R.F. choke 218 is connected between a source of DC bias voltage and the junction of coupling capacitor 206 and PIN diode 208.
In the transmit mode, the bias voltage is positive such that a DC bias current flows to ground through R.F. choke 218, PIN diode 208, transmission line 214 and PIN diode 216. This bias current switches PIN diodes 208 and 216 into the conducting state, such that the signal from transmitter 202 is coupled through PIN diode 208 to antenna 204, while PIN diode 216 shorts out the input of receiver 210 and one end of transmission line 214 to ground. It is well known that if a one-quarter wavelength transmission line is shorted at one end, the opposite end appears to be an open circuit. Therefore, because PIN diode 216 shorts one end of transmission line 214 in the transmit mode, the other end (the end connected to antenna 204) appears as an infinite impedance and substantially no energy from transmitter 202 flows through transmission line 214 to receiver 210.
In receive mode, the bias voltage is substantially at ground, thereby cutting off the DC bias current through R.F. choke 218, PIN diode 208, transmission line 214 and PIN diode 216. With no bias current through PIN diodes 208 and 216, both diodes are switched into a high impedance state. Because the PIN diodes are in a high impedance state, very little received signal energy is lost to ground through PIN diode 216, and transmission line 214 is terminated in its characteristic impedance by the input circuitry of receiver 210. Thus, the received signal at antenna 204 is coupled to the input of receiver 210 through transmission line 214 and coupling capacitor 212. In addition, no received signal energy is lost in the output circuit of transmitter 202 because the high impedance state of PIN diode 208 decouples the transmitter from antenna 204 in the receive mode.